This invention relates to a process for hydrogenating haloalkyl, halocycloalkyl or haloarylcarboxylic anhydrides to primary alcohols, in the liquid or vapor phase, in the presence of a solid rhodium or iridium catalyst, supported or unsupported, employed as the metal, metal oxide, or mixtures thereof.
Fluorine-containing alcohols are useful as solvents for a wide variety of organic compounds and are also useful as reagents for producing fluorine-containing esters of carboxylic acids, in which the alcohol moiety contains fluorine substituents. A commercially important fluorine-containing alcohol is 2,2,2-trifluoroethanol which can be used to produce the known anesthetic, isoflurane, CF.sub.3 CHClOCHF.sub.2.
Methods for producing fluorine-containing alcohols, such as 2,2,2-trifluoroethanol, usually involve the reduction of esters containing this alcohol moiety. For example, U.S. Pat. Nos. 3,314,987 and 4,072,726 (to Allied Chemical Corporation) describe the hydrogenation of such fluorine-containing esters over a copper oxide-based catalyst to produce fluorine-containing alcohols and similarly, U.S. Pat. No. 3,356,746 (to Allied Chemical Corporation) describes the hydrogenation of fluorine-containing esters over supported and unsupported ruthenium and palladium catalysts.
However, processes for hydrogenating halogen-containing alkyl carboxylic anhydrides directly to the corresponding primary alcohols are not well known since these anhydrides are generally more resistant to reduction than are the esters. This is particularly true for the lower members of the class such as trifluoroacetic anhydride and perfluoropropionic anhydride.
Anhydrides usually survive most catalytic hydrogenations. Prolonged, or vigorous reductions, however, may lead to hemiacetals, esters, acids, alcohols or even to hydrocarbons.
(RCO).sub.2 O+H.sub.2 .fwdarw.(RCHOH)OCOR PA1 (RCHOH)OCOR+H.sub.2 .fwdarw.RHC.sub.2 OH+RCOOH ##STR1## RCH.sub.2 OH+H.sub.2 .fwdarw.RCH.sub.3 +H.sub.2 O RCOOH+2H.sub.2 .fwdarw.RCH.sub.2 OH+H.sub.2 O ##STR2##
F. Swartz in Compt. Rend. 197, 1261 (1933) discloses reduction of trifluoroacetic anhydride in the liquid phase over platinum black at 20.degree.-40.degree. C. and 45-50 atm. Principal products of the reduction were CF.sub.3 CH.sub.2 OCOCF.sub.3, CF.sub.3 CH.sub.2 OH and CF.sub.3 CH.sub.3.
Zabolotskih et al. in USSR Pat. No. 514,803 issued May 1978 disclose vapor phase reduction of trifluoroacetic anhydride over a supported Pt or Ni catalyst resulting in predominant production of CF.sub.3 CH.sub.2 OH.
No mention is made in the above-described references of the use of rhodium or iridium-based catalysts for the heterogeneous hydrogenation of halogen-containing carboxylic anhydrides to a corresponding primary alcohol group.
Since the preparation of fluorine-containing alcohols by the direct hydrogenation of the corresponding fluorine-containing carboxylic anhydrides is potentially commercially attractive, what is desired is a direct one-step hydrogenation process applicable to a large class of fluorine-containing carboxylic anhydrides proceeding at reasonable rate under mild conditions of temperature and pressure.